Tanking operations must be reliably detected in various areas of technology. For example, in the chemical industry or in processing technology, it is of significance to detect tanking operations. Furthermore, it is advantageous in the area of motor vehicles to detect tanking operations. The detection of a tanking operation is of significance especially together with a test of the tightness of a tank-venting system.
Starting with the year 1996, the California Environmental Authority (CARB) requires an on-board diagnosis to check the operability of the tank-venting system in motor vehicles. In this connection, especially a test as to tightness of the tank-venting system should be performed.
A tank-venting system for a motor vehicle generally includes a fuel tank and a tank-venting valve which is connected to the intake pipe of an internal combustion engine so that, with the aid of underpressure in the intake pipe, fuel vapors can be drawn off by suction. Usually, the volume in the tank located above the fuel is drawn off via an adsorption filter mounted between the tank and the tank-venting valve. The adsorption filter in most cases is an active charcoal filter. This active charcoal filter adsorbs fuel in those time spans in which no suction from the intake pipe takes place, for example, when the engine is at standstill or when the tank-venting valve is held in the closed position because of the actual operating state.
U.S. Pat. No. 5,193,512 discloses a tank-venting system for a motor vehicle with a fuel tank. The tank-venting system includes an adsorption filter, which is connected to the fuel tank via a filter line, and which has a venting line. The tank-venting system also includes a tank-venting valve which connects the adsorption filter to the intake pipe of the engine via a valve line. A controllable shutoff valve is mounted in the venting line of the adsorption filter. In this way, the venting line of the adsorption filter can be blocked in a controlled manner.
The above makes it possible to carry out a method for checking the operability of the system in that the venting line of the adsorption filter of the system is shut off and the tank-venting valve of the system is opened. A measurement is made as to whether an underpressure has developed in the tank and, if this is the case, a conclusion can be drawn as to the operability of the system.
This method permits the tightness of the tank-venting system to be checked essentially by measuring the pressure difference between the pressure present in the tank and the ambient pressure. The system is viewed as being tight when an underpressure in the tank-venting system is maintained over a certain pregiven time span. However, if it is determined that the underpressure does not even build up or drops very rapidly, then a conclusion is drawn that a leak is present in the tank-venting system. A fault announcement to the driver of the motor vehicle is outputted and this fault announcement is stored in a memory. The check of the operability of the tank-venting system takes place with this method during idle and when the vehicle is at standstill.
The method for checking the operability of the tank-venting system functions satisfactorily during a proper operation of the vehicle.
However, it can happen that the vehicle is tanked in an impermissible manner with the engine running, for example, in order not to interrupt the operation of the climate control system in warm countries or not to interrupt the operation of the heating in very cold countries during tanking. If, when tanking the vehicle with the engine not switched off, that is, during idle of the vehicle, a diagnosis to check the operability of the tank-venting system is run through, a leak is diagnosed because of the open tank cap. A "leak" of this kind is, however, not to be attributed to damage to the tank-venting system and instead is attributed to an improper use of the vehicle. There is no interest to diagnose a temporarily occurring "leak" of this kind. Especially, this "leak" should not be stored in the fault memory of the diagnostic system.
For the above reason, a basic problem exists to detect tanking operations for motor vehicles and especially to store the above-mentioned fault announcements only when the tank is closed.
U.S. Pat. No. 5,193,511 discloses a tank cap switch utilized for control purposes. The switch opens when the tank is open and is closed when the tank is closed. A tank cap switch of this kind can be used for detecting tanking operations in vehicles. This, however, has the disadvantage that even this kind of tank cap switch must be checked with respect to operability as set forth in the requirements of the California Environmental Authority (CARB). Furthermore, a tank cap switch of this kind introduces additional costs and a leakage diagnosis would be continuously suppressed when, for example, it is neglected to close the tank.